Chemistry Reference
In-Depth Information
MeO
2
C
MeO
2
C
CO
2
Me
MeO
2
C CO
2
Me
catalyst
OAc
CO
2
Me
9.63
9.64
product using
Pd(PPh
3
)
4
9.65
product using
Mo(CO)
6
steric control
ML
n
charge control
9.66
Scheme 9.22
Br
9.67
OAc
CO
2
Et
MeO
2
C CO
2
Me
Mo(CO)
6
O
Mo(CO)
6
Br
Br
EtO
2
C
9
6
9
O
MeO
2
C
5:1 mixture of regioisomers
CO
2
Me
Scheme 9.23
CO
2
Et
CO
2
Et
CO
2
Et
catalyst
O
CO
2
Et
O
O
OAc
EtO
2
C
CO
2
Et
9.70
9
9
.
Mo(CO)
6
Mo(CO)
2
(NC
t
-Bu)
4
42 : 58
90 : 10
Scheme 9.24
while moderately hindered nucleophiles give mixtures. For molybdenum catalysis, the identity of the ligand
is critical. If some of the carbonyl ligands are exchanged for
t
-butyl isonitrile, the nucleophile once again
attacks the less-hindered terminus (Scheme 9.24).
32
-donor, and
therefore more effective at stabilizing the positively charged intermediate. Iridium catalysis can also lead
to this sense of regioselectivity, favouring attack at the more substituted terminus (Scheme 9.25).
33
This is because the isonitrile is a better
Even a
quaternary centre could be made, but needed a slightly better leaving group (Scheme 9.26).
A number of examples are known where the steric hindrance at the two termini is about the same, but
one terminus has a polar group, such as an alcohol or a carboxylic acid (Scheme 9.27). In these cases, the
nucleophile attacks the terminus of intermediate
9.80
most distant from the polar group.
34
Another example
may be found in Scheme 9.33.
